1. Field of the Invention
This invention relates to an ignition timing control system for internal combustion engines and, more particularly, to an ignition timing control system of this kind, which effects the ignition timing control in relation to control of slip of driving wheels of an automotive vehicle in which the engine is installed.
2. Description of the Related Art
Among conventional methods for electronically controlling the ignition timing of an internal combustion engine, there is widely known a method of controlling the ignition timing by the use of a spark ignition advance value set in dependence on operating conditions of the engine.
Conventional ignition timing control systems employing this method include an ignition timing control system which delimits a retard-side limit of a spark advance value corrected toward a retarded side in accordance with engine operating parameters etc., by means of a retard limiting value set in dependence On engine rotational speed to thereby prevent a misfire or a flashover due to sudden acceleration or sudden deceleration of the engine, as disclosed e.g. in Japanese Provisional Patent Publication (Kokai) No. 62-17666.
To suppress excessive slip of driving wheels of an automotive vehicle, there has been proposed by the assignee of the present application a slip control system which comprises slip detecting means for detecting slip of driving wheels, and control means responsive to an excessive slip of a driving wheel detected by the slip detecting means for interrupting the supply of fuel to an internal combustion engine installed in the vehicle (fuel out) or leaning the air-fuel ratio of a mixture supplied to the engine or retarding the ignition timing to thereby reduce the driving force or torque of the engine for suppression of the excessive slip (e.g. Japanese Provisional Patent Publication (Kokai) No. 1-290934).
In an internal combustion engine in general, the voltage required to be applied to the spark plug to cause sparking thereof (hereinafter called "spark voltage") changes as the air-fuel ratio of the mixture within the engine cylinder changes, even if engine operating parameters, such as intake air temperature remain unchanged. More specifically, as the air-fuel ratio of the mixture drawn into the engine cylinder becomes leaner, the ratio of an air amount contained in the mixture increases and the amount of fuel, which is conductive, decreases correspondingly. Consequently, the spark voltage becomes higher.
Therefore, if a retard limiting value defining the retard-side limit of ignition timing as disclosed in Japanese Patent Publication No. 62-17666, hereinabove referred to, which value is set to a constant value irrespective of whether or not an excessive slip occurs, is applied to control the ignition timing of an internal combustion engine having the above-mentioned slip control system, there is a problem of failure to spark by the spark plug and hence occurrence of a misfire or a flashover due to elevated spark voltage of the spark plug encountered when the air-fuel ratio is leaned upon detection of an excessive slip of a driving wheel of the vehicle.